The Bible explains much of what otherwise is unexplainable. The Bible says that these giants were around before and after Noah's flood. It seems that Goliath (slain by David, later King David) was one. I think also the Amalikites were of the Nephilim, which is why God told Israel to kill every one and not to spare any becasue the gene pool had been corrupted.

I think the existence of the Nephilim, the giants, can explain certain structures like the Stonehenge and the statues on Easter Island.

It seems that they're no longer around. Jude 1:6 says, "And the angels which kept not their first estate, but left their own habitation, [God] has reserved in everlasting chains under darkness unto the judgment of the great day." This seems to indicate that these mutant fallen angels who somehow changed their form to mate with human women, are permanently bound until the Final Judgment. That could mean we won't see these creatures anymore on earth.

However, in Matt. 24:27, Jesus said, "But as the days of Noah were, so shall also the coming of the Son of man be." This might leave open the possibility that during the Great Tribulation, these creatures could reappear. I wouldn't be surprised.

Jude 1:6 says, "And the angels which kept not their first estate, but left their own habitation, [God] has reserved in everlasting chains under darkness unto the judgment of the great day."

Matt. 24:27, Jesus said, "But as the days of Noah were, so shall also the coming of the Son of man be." This might leave open the possibility that during the Great Tribulation, these creatures could reappear. I wouldn't be surprised.

Don't forget

1 Pe 3:18 For Christ also died for sins once for all, the righteous for the unrighteous, that he might bring us to God, being put to death in the flesh but made alive in the spirit;

1 Pe 3:19 in which he went and preached to the spirits in prison,

1 Pe 3:20 who formerly did not obey, when God's patience waited in the days of Noah, during the building of the ark, in which a few, that is, eight persons, were saved through water.